Topics 1.H + 2.Acids and Bases 3.Definition of pH 4.Reversible reactions, equilibrium, mas action 5.HendersonpHasselbalch equation 6.Buffers. Buffer capacity

H+H+ Suppose chloride acid dissolved in water HCl H + + Cl - The entity H +, hydrogen stripped from the electron, is simply a proton, without electronic cloud, with dimensions at least 4 orders smaller than a real atom. Its strong electrical field Impedes a free existence. What really happens, upon dissolution of HCl in water is: HCl + H 2 O H 3 O + + Cl - H 3 O + H 2 O + H + HCl H + + Cl - [H + ]

Rutherford-Thompson atom: Dimensions m (Å) (Fermi)

Acids and Bases Brønsted-Lewy Concept (1923) Acid HA H + + A - Base B - + H + BH Arrhenius Concept (1890) Acid HA H + + A - Base COH C + + OH - Acid + base salt + water 2NaOH + H 2 SO 4 Na 2 SO 4 + 2H 2 O Water has amphoteric character 2H 2 O H 3 O - + H +

pH, reversible reaction, equilibrium, mass action Reversible Reactions – Rate constants - Equilibrium BA B + A K 1 k -1

Henderson-Hasselbalch equation HCl H + + Cl - K 1 k -1 At equilibrium

Buffers and Buffer capacity In a given pH, β is a function of pH and buffer concentration

Bibliography Bockris, J.O’M and Reddy, A.K.N.: Modern Electrochemistry. Plenum Press, Vol.1, Chap. 5. Protons in solution.

Questions 1.For a [H + ] of M to M, in steps fo 10 -3M, draw a plot of pH x [H + ]. 2.Consider 1 L of a solution of a buffer of pK=7.5 amd concentration of 10 mM. Starting with a buffer base concentration of 9,9 mM, add progressively a strong acid, in amounts of 0.05 mmol. At equilibrium draw the curve relating pH to the total amount of acid added. Where is the point of maximal buffering power? 3.Suppose a buffer if pK=7.0 in concentration of 5 mM. What are the concentrations of acid and base for buffering a solution at a pH of 6,0.

Medidas de pH I.Eletródios II.Indicadores fluorescentes Bibliografia Koryta, J.: Ion-Selective Electrodes Cambridge University Press. Vanysek, P.> The glass pH electrode.The Electrochemical Society Interface Lakowicz, J.R.: Principles of fluorescence spectroscopy. 2 nd ed., Fluwer Academy/Plenum Press

Electrochemical potential of a solute in a phase – Macroscopic view Thermal energy T (K) C2ø2C2ø2 M C: concentration, mol/l Ø: Electrical potential, V R= 8.3 J mol -1 K -1

Thermal energy – microscopic view Thermal energy Bezanilla simulation

Campos elétricos – forças elétricas Força elétrica – lei de Coulomb Campo elétrico Diferença de potencial elétrico +-+- Carga do e - 1,60* coul Constante de Faraday F=N A *e - = coul/mol

C2ø2C2ø2 M Membrane (M) Properties 1. Impermeable membrane 2. Membrane permeable to solutes 3. Membrane permeable to cations or to anions

Ion Exchangers – Glass Electrodes H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+

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